The present invention relates to a graphics display apparatus and particularly to such apparatus for selectively displaying one or more of a plurality of images that are input to the apparatus from common based computer input data.
A prior art graphics display system, such a described in U.S. Pat. No. 4,509,043, entitled METHOD AND APPARATUS FOR DISPLAYING IMAGES issued on Apr. 2, 1985, to Paula X. Mossaides and assigned to Tektronix, Inc., is capable of simultaneously or selectively portraying a number of different views on the same screen. For instance, the display apparatus may simultaneously display superimposed views of an automobile chassis, an automobile body, and an automobile steering column mechanism, or these views may be presented separately. Another example comprises the layout for a circuit board where the component parts and the wiring may be viewed separately or in properly superimposed relation. The various images are usually presented in different colors, or in different groups of colors, so that the separate portions of the structure may be identified even when a composite view is being presented.
The display apparatus ordinarily employs a color cathode-ray-tube capable of producing any number of different colors but operated in a given instance to display a small number of distinct colors identified with the various selectable images. Thus, an automobile body may be displayed in blue and green, while the chassis may be displayed in red and yellow.
The display is generated from a "pixel bit map" memory which at a given instant stores the color or colors of each pixel or each elementary part of the picture which is to be displayed by the cathode-ray-tube. Inputting to this pixel bit map memory is conveniently provided from the data bus of a computer wherein each pixel of the display is calculated or presented from "high level" data in the computer's memory. Thus, the computer's memory may store the line segments representing an automobile body as a plurality of vector lines, but this information is converted into pixels by the computer for storage in the aforementioned pixel bit map memory so that the pixel bit map memory may be scanned in normal raster fashion in order to present a TV-like display.
The input information from the computer to the pixel bit map memory suitably comprises a four-bit word representative of a given pixel and capable of defining sixteen different colors for that pixel, i.e. represented by the binary digits 0000 through 1111. Let us assume the word 0000 represents the absence of any information at the pixel point, while the binary number 0001 represents the color red, etc. Obviously any combination of sixteen colors can be identified by the four-bit word, including, of course, one identified for the total absence of a pixel. The scheme as thus far described would be capable of portraying one image in sixteen different colors.
For the purpose of presenting a plurality of different or separable images, the concept of separate "surfaces" comes into play, wherein one "surface" represents the automobile body, another "surface" represents the automobile chassis, etc. The body surface may be represented only in colors red and green, the chassis only in red and yellow, etc. In order to identify different surfaces, the four-bit word stored at each pixel location in the pixel map memory may be considered as divided into a number of subwords of a lesser number of bits. For instance, the pixel word stored for the pixel at the upper lefthand corner of the screen may include two lower order bits representing one surface, and two higher order bits representing another surface. In such case, the two lower order bits are capable of designating four colors by their various combinations, and the two higher order bits are capable of designating four (presumably different) colors by their combinations. Alternatively, with four bits in each pixel storage location, it is possible to designate four surfaces, one for each of the four bits. Other combinations are also possible.
For the eventual display, a color translator or color bit map comprising a separate memory is utilized to translate the stored indices for each pixel in the pixel bit map memory to the desired colors, and also to selectively show only one or more designated surfaces as desired.
A problem is presented in the prior art, however, when common based input data is employed to write to different surfaces. Thus, one input word may be received from the computer for writing to the red and green "surface" of the automobile body, while another word may be received which is to write information to the "surface" representing the automobile chassis. Both words are input on a common data bus and each is identically based, i.e. with the lowest ordered bit on the lowest ordered data bus conductor. Heretofore, it has been necessary when inputting certain patterns, vectors, shaded areas and the like, to require the computer to shift the input data or index a desired number of places in order to "write to" the proper "surface" in the pixel bit map memory. This shifting would frequently have to be accomplished for each data word, under the control of the computer software, thereby consuming excessive amounts of time in writing the desired information.